You can see why winning all four of these races would be a stretch, even though winning any one of them is doable, and probably likely. I’d say the probability of winning one of them is around 70 percent, two of them is around 50 percent, three of them is around 30 percent, while winning all four is maybe 10 percent. There are just too many things that need to break the GOP’s way to confidently predict a sweep.

To be sure, none of these seats has been won yet. Most people thought the GOP was a near-lock to take the North Dakota seat vacated by retiring Democrat Kent Conrad last cycle. Baucus’ Montana seat, in particular, is still no better than “lean Republican.” Democrats have a legitimate bench in the state, and the Republican bench is surprisingly weak (for more about the dynamics of that race, see here).

But Montana is now a race that the GOP probably should win; if it doesn’t we probably don’t even need to talk about the other four. (Think of it this way: We technically have to consider the possibility Republicans might win seats in Iowa and Michigan, but if they do that, the Senate has probably already gone Republican.) The Republican Party is significantly more likely to defeat three of the four incumbents discussed above than it is to defeat all four. For the first time this cycle, we can discuss a GOP takeover of the upper chamber in more than hypothetical terms.